Prior to the research carried out by the doctoral student at ULg, one of the stumbling blocks of the SSVEP-BCIs was the fact that they depended on checking the patient’s gaze, hence, the peripheral nerves and muscles. And yet, as mentioned above, some brain-injured patients no longer have this motor control. These systems, where the individual must direct their eyes in the direction of or at one of the target stimuli presented in their visual field, relate to what specialists call “overt” attention. Their applicability to patients whose visual control is altered or inexistent is a problem. Subsequently, Damien Lesenfants chose to design a system based on the alternative concept of “covert” attention, independent of motor control, where the subject focuses mentally on the target stimulus without having to move their eyes.

Hybrid SSVEP/Entropy system

In the system designed within the Coma Science Group, a panel is placed 30 cm from the subject’s head. It resembles a 7 x 7 cm draughtboard composed of alternate yellow and red square 1 x 1 cm electroluminescent diodes. The two types of stimuli (yellow diodes, red diodes) are therefore continuously in the subject’s visual field. However, the diodes flash at different frequencies depending on their colour: 10 hertz (10 times a second) for the red ones, 14 hertz for the yellow ones. “Twelve electrodes are placed at the back of the subject’s skull, where the visual areas of the occipital lobe are located”, Damien Lesenfants explains. “If they concentrate on a colour, neuronal activity in these areas will synchronise with the frequency of the flashes of the corresponding squares.”

The performance of this “covert SSVEP-BCI” was first tested on healthy patients. Like other systems of this type, the performance rate was 70 %, which is not enough to envisage applying the technique within the context of severe brain damage, i.e. locked-in syndrome, consciousness disorders. During the next stage of his research, Damien Lesenfants endeavoured to optimise the system by acting on several parameters (concentration duration, the features extraction algorithm, the number of harmonics, etc.). Furthermore, a new algorithm for the automatic selection of the electrode subassembly was elaborated, which is supposed to be the most relevant for each subject tested, regarding the features of their brain.

“Thanks to the optimisation of these different parameters, our covert SSVEP-BCI reached an 85 % performance rate in healthy patients, which allowed us to test it on patients with locked-in syndrome”, the ULg doctoral student points out. “In brain-injured patients, the performance levels are lower than in healthy patients. Also, based on the 60 to 70 % performance rates in the latter, we would only have been able to obtain results at statistical chance level in LIS patients.”

As regards locked-in syndrome, there were two aspects to the assessment of the interface: on the one hand, the system’s ability to detect the response to the command and, on the other hand, its capacity to serve as a means of communication between the patient and the outside world. The performances obtained were quite disappointing. The covert SSVEP-BCI only showed a response to the command in two patients out of six although they were all conscious, by definition, and it was only possible to establish communication with one patient out of four.